Complex compounds containing a metal of Group VIII of the Periodic Table as the central atom and, as ligands, P(III) compounds such as phosphines or phosphites, and optionally other complexing groups, have recently become increasingly important as catalysts. Thus the reaction, extensively practiced in industry, of olefins with carbon monoxide and hydrogen to yield aldehydes (hydroformylation) is carried out in the presence of catalyst systems composed of rhodium and triphenylphosphine. Catalysts based on complex compounds containing phosphines have also proven successful for the reaction of methanol with synthesis gas to give higher alcohols, especially ethanol and propanol (homologization). In such cases, the ligands are usually present in excess, so that the catalyst system is composed of complex compound and free ligand. Since these systems are soluble in organic media, the reaction is carried out in a homogeneous phase.
The reaction can also be carried out in the heterogeneous phase. This process variant is particularly convenient because it provides a simple way of separating the water-dissolved catalyst from the water-insoluble reaction product under mild conditions. The hydroformylation process described in DE 26 27 354 C2, for example, works on this principle. The system rhodium/sodium triphenylphosphine trisulfonate is used as the catalyst.
In addition to monophosphines, diphosphines are also used as constituents of catalyst systems in which the other component is a metal of Group VIII of the Periodic Table. For example, DE-A 40 40 315 relates to the preparation of aldehydes by reaction of monoolefins, non-conjugated polyolefins, cycloolefins, or derivatives of these classes of compounds, with carbon monoxide and hydrogen in the presence of rhodium/diphosphine catalysts. Sulfonated 2,2'-bis(diphenylphosphinomethyl)biphenyls or 2-(diphenylphosphinomethyl)-1-[2-diphenylphosphinomethyl)phenyl] naphthalenes are used as the diphosphines in this process. Together with the rhodium, they provide catalysts which are distinguished from the known rhodium/monophosphine systems by increased activity.